Volume 33, Number 10, October 1998

USGMACHINERY

Autoclaves for Glass Lamination

by Don Crockett

Glass lamination requires carefully controlled combinations of pressure and heat. For
windshield and architectural glass applications, the lamination process involves bonding
an interlayer of plastic between two lites of glass. The goal is to produce a clear lite
of shatterproof safety glass. To do this, the lamination process must form a strong bond
and remove air pockets from between the layers. With the special requirements of
todays glass lamination industry, autoclaves are often the best type of processing
equipment.

Over the years, autoclaves have been used to process a variety of materials. In
addition to glass lamination, they are used for bonding composite materials, vulcanizing
and curing rubber, laminating printed circuit boards, digesting pulp and paper, dewaxing
investment castings, enriching uranium and sterilizing hospital waste and other materials.

Autoclaves are industrial, ASME-coded pressure vessels with quick-opening access doors.
They are designed to apply pressure, heat and vacuum within a controlled environment.
Unlike laminating presses, autoclaves are able to apply pressure and heat uniformly,
regardless of the shape of the glass. This is important when laminating complex shapes,
such as wrap-around windshields.

Solving Limitations

Two factors limit the amount of glass that can be laminated: the size of the batch and
the length of the laminating cycle. One benefit that autoclaves have over presses is the
ability to laminate large batches of glass quickly. The amount of glass that can be
processed in a single lamination cycle is determined by the autoclaves work space,
which can be sized to accommodate large batches. Autoclaves provide the rapid heat up and
cool down needed for mass production.

Because of the high pressures and temperatures used for processing, autoclaves must be
certified to meet American Society of Mechanical Engineers (ASME) Pressure Vessel Code
standards. The thickness of the metal used to form the pressure vessel walls and the
integrity of welds are important for safe operation.

Door design is a crucial factor because of the continual need to open and close the
door, yet maintain an effective seal when the autoclave is pressurized. Smaller autoclaves
can be equipped with a simple T-bolt door that is opened and closed manually. But for
larger autoclaves, the sheer size and weight of the door make manual operation
impractical. Generally, larger autoclaves are equipped with hydraulic systems to open or
close and lock or unlock the door. Quick-opening doors typically feature a breech-lock
design that operates with a single turn of a locking ring. An airtight seal around the
door is initiated by an air-energized gasket. Once a seal has been achieved, it grows
tighter as the autoclaves internal pressure increases.

Custom Design Features

Although standard units are available, an autoclave is generally custom-designed to
meet the specifications of an individual process. Some industries require autoclaves with
pressure and temperature capabilities of 700 pounds-per-square-inch gauge (psig) and 1,100
degrees F. For glass lamination, pressures of 200 to 250 psig and temperatures of 350 to
450 degrees F are typical.

An autoclave is pressurized by filling it with compressed air or an inert gas such as
nitrogen or carbon dioxide. To shorten the lamination cycle, an air pressure accumulator
can be used to pressurize an autoclave very quickly. The type of heating system that is
specified often depends on what energy source is most readily available at the facility
where the autoclave will be used. Options include electric-resistance heat, gas-fired
heat, fuel-oil-fired heat and dry heat from steam or hot oil exchangers.

Uniform distribution of pressure and temperature is needed to form a strong bond
between the laminated sheets of glass. Because its entire work space is pressurized, an
autoclave automatically applies pressure uniformly across the surface of the glass or
other material being processed. But for uniform temperature distribution, a fan and
ductwork are required to circulate heated air throughout the pressure vessel. How the
glass is loaded will determine the design of the autoclaves air circulation system.
When glass is stacked vertically, a bottom-to-top airflow is most effective.

Heating and fan systems are usually located at the closed end of an autoclave. If the
fan assembly is mounted inside the autoclave, it must be equipped with a cooling coil to
protect it from the high temperatures applied during processing. Fan assemblies that are
mounted externally require a pressure seal where the drive shaft penetrates the wall of
the autoclave.

Autoclave controls range from simple microprocessor-based systems to the sophisticated
systems that incorporate programmable logic controllers and personal computers. All are
designed to provide precise control of the autoclave functions involved in a lamination
cycle: heat-up rate, soak period, cool-down rate, pressurization and depressurization.
More sophisticated control systems can be programmed to store a large number of ramp/soak
profiles so that different lamination processes can be run without reprogramming the
autoclave between each cycle.

Control systems also provide a range of data logging capabilities. By monitoring all
thermocouple and pressure vessel operating conditions, computer-based systems can furnish
continually updated historical trend displays. This operating data can be stored and used
to generate reports to verify the quality of each lamination cycle.

Autoclaves can be supplied with material handling equipment to help load and unload the
glass that is being laminated. To accommodate carts and trolleys, specially designed ramps
and tracks can be built in. In some cases, conveyor systems have been installed to
eliminate manual loading and unloading. To simplify maintenance between cycles, cleanout
ports can be included for draining any excess plasticizer that builds up inside the
autoclave.

Safety Measures

Autoclaves should be inspected regularly, and any modifications need to be tested to
make sure that they meet code standards. Because of the high levels of pressure and heat
generated inside an autoclave, safe operating procedures should always be followed. Modern
autoclaves are equipped with redundant safety systems to prevent them from being
accidentally opened while they are still pressurized. Manual and electro-pneumatic lock
pins must be in place before pressurization can begin. Once the autoclave has been
pressurized, a zero-pressure switch prevents the door from being opened. A warning whistle
sounds if there is an attempt to bypass these safety devices and open the door before the
autoclave has been fully depressurized.